The present invention relates to frequency-swept oscillator control and, more particularly, to a novel method and apparatus for automatic correction of initial frequency and/or sweep rate of a chirped-frequency-swept-keyed oscillator.
In co-pending allowed application Ser. No. 301,706, filed Sept. 14, 1981, assigned to the assignee of the present invention and incorporated by reference herein in its entirety, a method for digital data communication, over hostile media, utilizing chirped-frequency-shift-keyed signals is described and claimed. While this chirped-frequency-shift-keyed (CFSK) modulation system has proven to be a highly reliable technique for communication in certain circumstances, such as over the power circuits in a building and the like, the system requires chirped oscillators at separated locations to be synchronized in time and means for time synchronization is described and claimed in the aforementioned co-pending application. In some applications, the chirped oscillators must also be frequency synchronized. A method and means for automatically controlling the frequency of the chirped oscillators in such a system, particularly over a relatively long time interval (without manual adjustment thereof) is highly desirable to achieve system stability. Similarly, it is highly desirable to automatically control the sweep rate at which the oscillator frequency is chirped during each time interval in which a bit of digital data is transmitted.
It is known to automatically frequency control an oscillator having a continuously or slowly changing output frequency, such as the carrier frequencies found in AM and FM modulation systems. The frequency of a station oscillator is thus controlled to make it follow an incoming signal which is either a constant or continuously varying frequency. Such automatic frequency control approaches are not applicable in a system having CFSK modulation, as a carrier is not available for use as a reference and both the starting frequency and the sweep rate are desirably controlled. Accordingly, a method and means for providing initial frequency and/or sweep rate control in the swept oscillator of a CFSK modulation system is highly desirable.